https://doi.org/10.1351/goldbook.09346
Photoelectron peaks in a spectrum resulting from photoemission induced by characteristic minor X-ray lines associated with the X-ray spectrum of the anode material.
Example: \(\ce{K}\upalpha\!^{\prime}\), \(\ce{K}\upalpha\!_{3,4}\), \(\ce{K}\upalpha\!_{5,6}\), and \(\ce{K}\upbeta\) are all minor X-ray lines.